1. CS 536 / Fall 2017 Introduction to programming languages and compilers
Aws Albarghouthi

2. About me PhD at University of Toronto
Joined University of Wisconsin in 2015
Part of madPL group
Program verification
Program synthesis

3. About the course We will study compilers
We will understand how they work
We will build a full compiler
We will have fun

4. Course Mechanics Home page: http://cs.wisc.edu/~aws/courses/cs536-f17/
Workload:
6 Programs (40% = 5% + 7% + 7% + 7% + 7% + 7%)
10 short homeworks (20%)
2 exams (midterm: 20% + final: 20%)

5. A compiler is a recognizer of language S a translator from S to T
a program in language H

6. front end = recognize source code S;
map S to IR
IR = intermediate representation
back end = map IR to T
Executing the T program produces the same
result as executing the S program?

7. Phases of a compiler P2 P3 P1 P4, P5 front end back end P6 Symbol
table
P4, P5
front end
back end P6

8. Scanner (P2) Input: characters from source program
Output: sequence of tokens
Actions:
group chars into lexemes (tokens)
Identify and ignore whitespace, comments, etc.
Error checking:
bad characters such as ^
unterminated strings, e.g., “Hello
int literals that are too large

9. a = 2 * b + abs(-71) Example scanner
ident
(a)
asgn
int lit
(2)
times
ident
(b)
plus
ident
(abs)
lparens
int lit
(71)
rparens
minus
Whitespace (spaces, tabs, and newlines) filtered out
a = *b+
abs ( -
71)
The scanner’s output is still the sequence
ident
(a)
asgn
int lit
(2)
times
ident
(b)
plus
ident
(abs)
lparens
int lit
(71)
rparens
minus

10. Parser (P3) Input: sequence of tokens from the scanner
Output: AST (abstract syntax tree)
Actions:
groups tokens into sentences
Error checking:
syntax errors, e.g., x = y *= 5
(possibly) static semantic errors, e.g., use of undeclared variables

11. Semantic analyzer (P4,P5)
Input: AST
Output: annotated AST
Actions: does more static semantic checks
Name analysis
process declarations and uses of variables
enforces scope
Type checking
checks types
augments AST w/ types

12. Semantic analyzer (P4,P5)
Scope example: … {
int i = 4;
i++; }
i = 5;
out of scope

13. Intermediate code generation
Input: annotated AST (assumes no errors)
Output: intermediate representation (IR)
e.g., 3-address code
instructions have 3 operands at most
easy to generate from AST
1 instr per AST internal node

14. Phases of a compiler P2 P3 P1 P4, P5 front end back end P6 Symbol
table
P4, P5
front end
back end P6

15. a = 2 * b + abs(-71) Example scanner parser ident (a) asgn int lit (2)
times
ident
(b)
plus
ident
(abs)
lparens
int lit
(71)
rparens
minus
parser

16. Example (cont’d) semantic analyzer Symbol table a var int b var int
abs fun int->int

17. Example (cont’d) code generation tmp1 = 0 - 71 move tmp1 param1
call abs
move ret1 tmp2
tmp3 = 2*b
tmp4 = tmp3 + tmp2
a = tmp4
code generation

18. Optimizer Input: IR Output: optimized IR Actions: Improve code
make it run faster; make it smaller
several passes: local and global optimization
more time spent in compilation; less time in execution

19. Code generator (~P6) Input: IR from optimizer Output: target code
This slide does not exactly match what will be done in Program 6. In CS 536, we are building a simple compiler that does not have an optimization phase. Thus, instead of translating the (annotated) AST to intermediate code and then translating the intermediate code to target code, P6 goes directly from the (annotated) AST to target code

20. Symbol table (P1) Compiler keeps track of names in
semantic analyzer — both name analysis and type checking
code generation — offsets into stack
optimizer — def-use info
P1: implement symbol table

21. Symbol table Block-structured language Java, C, C++ Ideas:
nested visibility of names (no access to a variable out of scope)
easy to tell which def of a name applies (nearest definition)
lifetime of data is bound to scope

22. Symbol table int x, y; void A() { double x, z; C(x, y, z) } void B() {
block structure: need
symbol table with nesting
int x, y;
void A() {
double x, z;
C(x, y, z) }
void B() {
C(x, y, z);
implement as list of hashtables